Stability of photographic paper emulsion coatings



May 3, 1960 J. SOTTYSIAK STABILITY OF PHOTOGRAPHIC PAPER EMULS IONCOATINGS Filed Aug. 25, 1955 DENSITY LOG EXPOSURE DENSETY V F l LOGEXPOSURE INVENTOR JOSEPH SOTTYSIA K United States Patent Q ce "STABILITYOF PHOTOGRAPHIC PAPER EMULSION COATINGS Joseph So ttysiak, B in ghamton, assignor to General Film gorporation, New York, N.Y., acorporation of Delaware Application August 25, 1955, Serial No. 530,6156 Claims. (c1. 96- 110) Tli'e present invention relates to photographicpaper coatings and, specifically, to the improvement of gradationstability of photographic paper coatings prepared Q frorn clilo'robromide type photographic paper emulsions,

characteristiccurves are plotted for the coated emulsion according tothe well-known method of Hurter and Driffield (so-called H and D curve).

Photographic emulsions are generally coated on -a paper base in variousgrades of contrast, i.e., soft, 'mehard "and extra hard. The spacingbetween the gradations is so de'signatedthatphotographic prints can bemade by the'usual methods of, exposure'to light from neg'atives'of'varyin'g contrast. The photographic paper manufacturer attempts tostabilize'thcse contrast spacings so they will maintain themselvesthroughout the life of "thepaper. In most'ca'ses, the commercial qualityof a photographic [paper is expected to continue for a period of 'upfto,two years, but considerable change occurs throughout the life of thepaper. Various methods are used to obtain and maintain the spacingbetween con- 'trasts. A common one is to coat the photographic emulsion'at the highest limit of its contrast specification so "that, upon agingand when ready for use, it will have leveled ofiat tlie designatedgradation. This practice 'hasmany obvious deficiencies.

The other alternative is for the consumer to adjust "processingconditions so as to compensate for shifts in paper quality. This is notsatisfactory since he may have fvari qus contrasts or different coatingages. By my'invention intend to, prepare a photographic paper emulsionwhich 'is considerably more stable toward change with aging by use ofstrontiumcliloride in the preparation 'of chloride-bromide typeemulsions and Whilecm- ,ployin g a critical quantityofstrontiurnchloride during silver halide precipitation. The preparationof such emulsions and the emulsion so prepared constitutetlie purposesand objects of the present invention.

The "literatur pertaining to photographic emulsion making technique isquite meager. The use of certain ehemicalsgduripg the precipitation ofthe silver halides in the carrier colloid gives, photographic resultstotally unpredictable from a normal knowledge of their chem "icalproperties. s "Reference to'theu'se of strontium chlorideis-giv'en inBakers Photographic Emulsion T echnique7"where strontium and lithiumchloride are used as a source of chloride ion in forming silver chloridein a collodion type chloride printing out paper. Reference in this samebook is made to the fact that halides of barium, cadmium and othermetals give warmer toned emulsions than do those of 2,935,405 PatentedMay'3, 1960 2 potassium, sodium and ammonium. No other reference wasfound in the literature for the specific use of strontium chloride. Asstated, I do not propose to use strontium chloride the knownrelationship nor as a means of obtaining warmer tones, but'as agradation stabilizer.

I procure stabilization of gradation through the use of strontium"chloride in combination with sodium chloride in the presence of otherhalides during the precipitation of silver halide in a carrier colloidsuch as gelatin to form a photographic paper emulsion. In thepreparation of unwashed chloro bromide emulsions, it is normal practiceto use halide salts such as potassium bromide and sodium chloride and toform, by chemical interaction with silver nitrate in the presence of thecarrier colloid such as gelatin, a silver bromide-silver chloridecrystal complex. A small quantity, usually less than 10% of the bromide,of potassium iodide is added to give added sensitivity to the emulsion.Sodium chloride and potassium bromide are normally used as sources ofhalide ions because of their abundance, availability and favorablecost.Other metal halides can be used as sources of chloride ions. Strontiumchloride in this invention is not; used merely as a source of chlorideions, but as an adduct to the sodium chloride-potassium bromidecombination. It is possible without strontium chloride to manufacture asuitable chloro-bromide emulsion but, surprisingly with the addition ofcritical amounts of strontium chloride, a' markedly superior product isobtained.

A chloro broinide'emiilsion is prepared by interacting in the customaryway in the presence of gelatin an aqueous solution or potassium bromide,sodium chloride and anarnoiiritofpotassium iodide, not 'to exceed 10%of'tl'le potassium'bremide, 'with silver nitrate so as not to completelyprecipitate all of the halide ions, but to leave a'slight excess, notmore than 15% of the total.. This excess of halide in an unwashedemulsion is beneficial in stabilizing the emulsion against fog. At theproper temperature (SO- C.) and with the appropriate silver nitrateaddition, a photographic emulsion is obtained which is subsequentlycoated on a paper base. This emulsion has the characteristics of Figure1 of the accompanying self-explanatory drawing which illustrates theinvention by sensitometric'curves prepared in the manner of Hurter andDrifiield.

It is-foundthat When the chloride salt concentration is changed so as toinclude the chlorides of strontium and sodium in a definite ratio of molpercentages, an

emulsion is obtained which, when coated on a paper base, gives a-product with the characteristics of Figure 2. F

in each figure refers to the freshly coated paper exposed in asensitometer togive a characteristic H and D curve.

0, on the other hand, refers to the same coating exposed in asensitometer after a four-day accelerated oven aging test effected at F.(60 C.) and a relative, humidity of 20-25% This'type of test treatmentcorresponds substantially to a normal natural aging of approximately oneyear. It will be observed that in Example l, there is considerable lossin gradation after aging, whereas in Example II, "there is relativellittle less in gradation with aging;

It 'is necessary to operate within certain limits of halide salts toobtain the desired improvement. This range extends from a ratio of moreicem of (a) 12.0 of sodium chloride "to one ofstrontium chloride to('b) 2.7 of sodium 'chloride 'to one or strontium chloride.Concentrations lower than the 12/ 1 ratio have 'littleor no effect inimproving gradational stability. Concentrations in a ratio higher than2.7/l give an emulsion with poor photographic properties. Thus, theemulsions are diflicult to coat on a paper base and after aging, exhibitpeculiar properties in normal photographic developer solutions.

The particular reasons for the improvement obtained have not beendetermined and are not easily explainable. Two theories presentthemselves but whether either is applicable is unknown. It is impossibleto determine which of the halides present reacts with the silvernitrate. It is barely possible that the strontium chloride does notreact or reacts only in part. This would mean that free strontiumchloride is present in the emulsion, it being emphasized that theemulsion being a paper emulsion is unwashed.

It is noted, on the other hand, that in addition to the strontiumchloride there is present potassium bromide, sodium chloride andpotassium iodide; The possibility, therefore, exists that, upon reactionof the halides with silver nitrate, a peculiar heretofore unattainablecrystal lattice is produced. However, regardless of the theory, the factremains that by utilization of the various halides including strontiumchloride and the particular ratio pointed out, new and unexpectedresults are obtained from the standpoint of gradational stability.

The following examples will serve to illustrate the invention but it isto be understood that the invention is not restricted thereto.

Example I The following basic formula defines a composition with thelowest concentration of strontium chloride. The strontium chloride usedis the normally available crystalline hexahydrate', i.e., SrCl .6H O.The mol percent is calculated on the actual SrCl present in the crystal.

I. Potassium bromide --g..- 14.40 Sodium chloride --g-.. 8.40 Strontiumchloride .g 3.20 Potassium iodide ..g 0.96

II. Gelatin g 98.20 Water 690.00

III. Silver nitrate ....g 36.00 Water c..- 360.00

The same basic formula was used as in Example I, excepting that theratio of salts was as follows:

I. Potassium bromide 9.95 Sodium chloride 7.80 Strontium chloride 12.70Potassium iodide 0.96

Example Ill The same basic formula was used as in Example I, exceptingthat the ratio of salts was as follows:

1. Potassium bromide 9.95 Sodium chloride 7.85 Strontium chloride 6.25Potassium iodide 0.96

Example IV An ordinary emulsion was prepared for use in comparison inthose of Examples I, H and III with the following ratio of salts;

G. I. Potassium bromide 9.95 Sodium chloride 12.50 Potassium iodide 0.96

The following table depicts the differences that exist on a mol percentbasis with the aforestated ratio of sodium chloride to strontiumchloride:

M01 Percent Example Ratio, NaOl/SrOI, N aOl SrOlz Gamma Example FreshAged Change -o. 11 -0. 10 O. 12 -o. 25

An examination of the table shows that by my invention the changes whichoccur upon aging have been greatly minimized. Thus, the emulsions ofExamples I, II and III have considerably better gradation stability thanemulsions prepared according to usual practice.

I claim:

1. The process of producing an unwashed silver halide paper emulsionhaving improved gradation stability which comprises precipitating in acolloidal carrier therefor, a mixed silver halide by reacting silvernitrate with an aqueous solution of potassium bromide, sodium chloride,potassium iodide and strontium chloride in the presence of saidcolloidal carrier, the molar ratio of sodium chloride to strontiumchloride being between 2.7 and 12 of the former to 1 of the latter.

2. The silver halide emulsion made by the process of claim 1. I

3. The process as defined in claim 1 wherein the colloidal carrier isgelatin.

References Cited in the file of this patent UNITED STATES PATENTS Crouseet al May 28, 1946 OTHER REFERENCES Baker: Photographic EmulsionTechnique (2nd edition), 1948, American Phot. Publishing Company,Boston, page 251. Copy in Division 67. p

1. THE PROCESS OF PRODUCING AN UNWASHED SILVER HALIDE PAPER EMULSIONHAVING IMPROVED GRADATION STABILITY WHICH COMPRISES PRECIPITATING IN ACOLLOIDAL CARRIER THEREFOR, A MIXER SILVER HALIDE BY REACTING SILVERNITRATE WITH AN AQUEOUS SOLUTION OF POTASSIUM BROMIDE, SODIUM CHLORIDE,POTASSIUM IODIDE AND STRONTIUM CHLORIDE IN THE PRESENCE OF SAIDCOLLOIDAL CARRIER, THE MOLAR RATIO OF SODIUM CHLORIDE TO STRONTIUMCHLORIDE BEING BETWEEN 2.7 AND 12 OF THE FORMER TO 1 OF THE LATTER.